In order to reduce heat losses, the stator, the rotor, and the gap therebetween are maintained in a vacuum, with the stator and the rotor each requiring a nigh vacuum, of about 10.sup.-4 milibars, while the vacuum in the gap therebetween my be considerably less, since its main role is to reduce friction losses. Permanent pumping is provided on the stator in order to maintain the high vacuum. The motor is preferably in a sealed vacuum. However, when the machine is cooled down, a high degree of differential contraction takes place between the hot part of the rotor in the vicinity of the gap and its cold part which needs to be brought down to about 4.degree. K., for example by a flow of helium at above its critical pressure. This requires the use of members capable of absorbing the differential contraction of the stator and the rotor, thereby complicating its construction. Finally, given the differential contraction between the rotor and the stator, the gap therebetween must be rather large.
Preferred implementations of the present invention provide a synchronous machine having a cooling system and a rotor which are of simpler construction and its gap may be smaller.